1. Field of the Invention
The present invention relates generally to a method for controlling regenerative braking in an electric vehicle, and more particularly to a method for controlling regenerative braking in an electric vehicle in which a braking force does not suffer from a loss when a data communication using a control area network (CAN) standard fails between a regenerative braking torque controller and a hydraulic pressure braking torque controller.
2. Description of the Background Art
An electric vehicle is a vehicle having one or more electric motors for propulsion. In the electric vehicle, electrical energy stored in a battery is used in driving the electric motor. Some or all of electrical energy in return is used to drive wheels of the electric vehicle. The electric motor has the ability to convert kinetic energy back into electricity through regenerative braking as a generator.
There are two types of electric vehicles commonly available: a pure electric vehicle, which converts chemical energy to electrical energy in batteries, and a hybrid electric vehicle (HEV), which converts chemical energy to electrical energy via an internal combustion engine and a generator.
The electric vehicle is hereinafter meant to include whatever vehicles utilizing chemical energy stored in rechargeable battery packs to drive the electric motor which in return drives the wheels of the electric vehicle. That is, the electric vehicle uses electric motors instead of, or in addition to, an internal combustion engine. The electric motor not only drives the wheels of the electric vehicle, but also generates electricity, which is to be stored in the battery during braking action operation. This is known as regenerative braking. Through regenerative braking process, the electric motor generates electricity as well as a braking force during braking action operation. That is, a part of the kinetic energy, which would otherwise be lost to heat during braking action operation, is converted into and stored in the battery as electrical energy for prospective us in future.
From this aspect, regenerative braking in the electric vehicle serves to increase fuel mileage and to reduce harmful exhaust gas.
The braking force, generated through regenerative braking, is applied to the wheels connected to the electric motor. Hence, the electric vehicle is additionally equipped with a hydraulic braking system, which generates the braking force using hydraulic pressure. This is because combination of the regenerative braking system and the hydraulic braking system in the electric vehicle makes vehicle dynamics control guaranteed more effectively.
As shown in FIG. 5, a total torque, obtained when a driver pushes down on the brake pedal during normal breaking, is a sum of the hydraulic pressure breaking torque and the regenerative braking torque. This makes it possible for the hydraulic pressure braking force to compensate for loss of the regenerative braking force when regenerative braking operation fails. The hydraulic pressure braking produces heat by forcing a stationary brake pad against a rotating disc. The regenerative braking force serves to generate electricity when reducing speed of the driving motor, by using the motor as a generator. The generated electricity is stored in the battery.
In a case when the regenerative braking controller fails to cooperate with the hydraulic pressure braking controller to produce the braking force, the driver would feel additional application of braking force, that is, earlier application of braking force, while the regenerative braking system works during braking.
To prevent this phenomenon, data communication has to be established between the regenerative braking controller and the hydraulic pressure braking controller to maintain relationship between the two braking controllers.
However, when the data communication fails between the two braking controllers, the hydraulic pressure braking torque sharply increases and the regenerative braking torque sharply decreases to level “0”, as the graph in FIG. 6 indicates. Therefore, on the base of the general concept that the total braking torque is equal to hydraulic braking torque and the regenerative braking torque, the total torque 62 summated by hydraulic braking torque 60 and regenerative braking torque 61 become lower than the previous total braking torque 64 during the data-exchange failure. This phenomenon makes the driver feel a lack of braking force.
To improve this lack of braking force a conventional braking torque control system, as shown in FIG. 7, increases the regenerative braking torque gradually and increases the hydraulic pressure braking torque gradually until braking ends when the data communication fails between the two braking controllers. This prevents the braking force from being reduced, and enables the driver to feel a normal braking force when the driver steps on the brake pedal.
However, gradual reduction in the regenerative braking torque brings about corresponding reduction in generation of electricity, thus making the kinetic energy converted less to electrical energy. Furthermore, it is practically impossible to exactly match increase in the hydraulic pressure braking torque with decrease in the regenerative braking torque without decreasing the total braking torque. This mismatch makes the driver feel loss of the braking force when the driver steps on the brake pedal.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.